In primary knee joint replacement surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure; a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component respectively. In a typical primary knee joint replacement surgery the ligaments and tendons are sufficiently intact to control the movement of the knee.
The femoral component is placed on a patient's distal femur after appropriate resection of the femur. The femoral component is usually metallic, having a highly polished outer condylar articulating surface, which is commonly J-shaped.
A common type of tibial component uses a tray or plateau that generally conforms to the patient's resected proximal tibia. The tibial component also usually includes a stem which extends at an angle to the plateau in order to extend into a surgically formed opening in the patient's intramedullary canal. The tibial component and tibial stem are both usually metallic.
A plastic or polymeric (often ultra high molecular weight polyethylene or UHMWPE) insert or bearing fits between the tray of the tibial component and the femoral component. This tibial insert provides a surface against which the femoral component condylar portion articulates—moves in gross motion corresponding generally to the motion of the femur relative to the tibia.
In some knee prostheses, the tibial insert also engages in motion relative to the tibial tray. Such motion can be translational and-or rotational sliding motion relative to the tibial plateau. In other types of knee prostheses with tibial inserts, the tibial inserts can engage in other types of motion relative to the tibial plateau and-or femoral component.
Revision surgery is required when the primary prosthesis fails. In most revision cases additional stabilization and structure are necessary to compensate for loss of bone and soft tissue. For example, the femoral and tibial components may be thicker to make up for the loss of bone. The femoral component may include a stem, which generally extends at about six degrees from perpendicular from the base portion of the femoral component in order to extend into a surgically formed opening in the patient's intramedullary canal. In order to provide increased stabilization, a box may be provided on the femoral component and a mating post on the tibial component, creating what is called a constrained knee replacement.
In some cases, the loss of soft tissue in the knee requires the use of a linked or hinged knee prosthesis. The three most common indications that a hinged knee is necessary are: (1) in an increasing number of revision cases, the patient loses too much bone and soft tissue to use a constrained knee; (2) an oncologist may be forced to resect a large portion of a bone in order to remove a tumor; and (3) in trauma applications, often the distal femur or proximal tibia has been crushed and must be replaced.
Early hinged knees were fixed, allowing no internal-external rotation. These early hinges had a history of loosening because their fixation could not adequately handle applied forces. Rotating hinges decreased this failure because these rotating hinges minimized internal-external rotational torque. Hinged knee systems provide a physical link of two components with an axle, such that all medial-lateral and anterior-posterior stability is provided by the prosthesis. These systems also address various degrees of bone loss. During normal articulation, the pivot axis for the axle is fixed in the anterior-posterior and superior-inferior directions, so that when the knee is flexed or extended about the axle the center of contact between the femoral and tibial components is fixed. This prevents roll-back.
A major concern with hinged knees is simulating the movement of a natural knee joint. The movement of a natural knee joint has three translations: anterior-posterior, medial-lateral, and inferior-superior and three rotations: flexion-extension, internal-external, and abduction-adduction. The movements of the knee joint are determined by the shape of the articulating surfaces of the tibia and femur and the orientation of the major ligaments of the knee joint, including the anterior and posterior cruciate ligaments and the medial and lateral collateral ligaments as a four linkage system. Knee flexion-extension involves a combination of rolling and sliding of the femur on the tibial plateau called femoral roll-back. In roll-back during flexion, the center of contact between the femur and the tibial plateau moves posteriorly, which allows increased ranges of flexion and increased efficiency of the extensor mechanism.
Current hinged knees typically allow both hinge-over in the flexion-extension direction and internal-external rotation, but do so by flexing about a fixed pivot axis that eliminates roll-back. Some hinged knee designs, on the other hand, have hinge mechanisms that allow roll-back, but do not control roll-back. No known hinged knee systems both allow and control roll-back.
During pre-op planning the extent of bone and soft tissue damage is not always discernable. Since surgical preference typically is to use the least intrusive procedure, a revision with a constrained prosthesis, as opposed to a hinge knee, is preferred. If, during surgery, it becomes apparent that a hinge knee is necessary, it would be preferable for the hinge to be part of an integrated system so the surgeon can proceed with minimal interruptions. Current hinged systems are stand alone, so that if the surgeon plans to use a constrained knee but realizes during surgery that the added constraint of a hinged knee is required, the surgeon cannot switch to a hinged knee during the procedure. Rather, the surgeon typically has to start another procedure resulting in longer operating times and greater risk to the patient. Additionally, current hinged knees require the surgeon to remove a large portion of the patient's bone in order to allow proper implantation.
Current hinged knee systems require a considerable amount of assembly during surgery in order to ensure that the various components are properly sized and connected. Such assembly takes time, is tedious and prone to error, and averts the surgeon's attention from more critical matters directly related to the health of the patient.
Thus, there is a current need for a hinged knee prosthesis that provides natural kinematics without excessive bone removal. There is also a need for a hinged knee system that is compatible with existing total knee replacement systems. Finally, there is a need for a hinged knee system that requires less assembly during surgery.